The monitoring and control of concrete “slump” in ready-mix delivery trucks has been described in a number of published patent documents, which are summarized below and incorporated herein by reference.
In U.S. Pat. No. 4,008,093, Kitsuda et al. disclosed that the slump property of a concrete mix could be controlled by measuring electrical energy required for rotating the mixing drum and allowing the truck operator to adjust slump by adding water to maintain it within a certain slump range, thereby making longer transportation of concrete by truck mixer possible.
In U.S. Pat. No. 5,713,663, Zandberg disclosed that concrete slump could be controlled by monitoring torque on the hydraulic drive of the truck mixing drum and automatically adding a liquid component to adjust the concrete mix to a desired slump, as detected by a minimum torque loading on the mixing drum.
In U.S. Pat. No. 6,484,079, Buckelew et al. disclosed that the slump control of Zandberg et al. could be remotely monitored. The status of the delivery truck could be reported and tracked using wireless transmission and global positioning units.
In application Ser. No. 09/845,660 (Publication No. US2002/0015354A1), Buckelew disclosed that continuous monitoring of slump, using GPS positioning systems, could help to detect whether the truck operator or construction foreman added water to make the concrete easier to spread (paragraphs 0005-0006). This unauthorized addition of water could work detriment to the concrete mix by decreasing compressive strength. Thus, Buckelew taught that slump be monitored numerous times during delivery, and the slump data downloaded by wireless transfer at the installation site.
In U.S. application Ser. No. 10/599,130 (Publication No. 2007/01856A1), Cooley et al. disclosed a system for calculating and reporting slump in a truck drum that had a hydraulic sensor coupled to the hydraulic drive and a rotational speed sensor connected to the drum. Both sensors were connected to a wireless communication system. This permitted modifications to be made to the truck operation during the delivery service.
The monitoring of concrete slump involves calibrating the values obtained from the hydraulic or electrical sensor on a mixing truck and correlating these with slump values obtained using a standard slump cone test. In a standard slump cone test, a 12-inch truncated cone containing fresh concrete is removed to permit the concrete to drop, and the vertical height drop of the concrete is measured (ASTM C143-05).
The present inventors believe that the slump of a concrete mix does not provide an accurate indication of its segregation resistance. This resistance to segregation refers to the ability of the concrete mix to cohere with uniform consistency such that separation of component solids is avoided. Concrete is a suspension made from mixing water, cement, and aggregate (e.g., sand, crushed gravel). The denser material (usually aggregate) tends to sink downwards when mixing stops. The turn-screw effect of the blades or paddles mounted inside the rotating drum of the truck can exacerbate segregation by pushing the aggregate in one direction along the axis of drum rotation.
Segregation can lead to diminishment of the concrete mix “pumpability” (i.e., ability to be conveyed through a conduit) as well as of its “finishability” (i.e., ability to provide smooth but dense outer surface). On the other hand, it is important to control cohesiveness so that it does not become excessive to the point of hindering ease of pumping or finishability.
In U.S. Pat. No. 6,227,039, Te'eni stated that “different concrete mixes can exhibit equal workabilities (slump) when measured by different techniques and yet can possess totally different rheological properties relating to their suitability for commonly required applications,” such as pumping (col. 2, II. 14-20). He disclosed a shearing sensor unit having vibrating and shear-inducing devices and sensors for measuring stress and acceleration (FIG. 1; col. 6-7). A piston pushed the shear box downwards, forcing the concrete sideways out of open ends of a U-shaped shear box, where multiple sensors monitored the resistance to movement of the piston rod (col. 7, I. 59—col. 8, I. 9). The shear box could be mounted within the truck drum to transmit data wirelessly to a mixing plant so that a rheological profile could be generated based on workability, stress state sensitivity, stress distribution, shear rate sensitivity, vibration decay, vibratability, pumpability, and deformability (col. 8, II. 59—col. 9, II. 39).
However, mounting the U-shaped box shear-sensing device of Te'eni within the mixing drum of a concrete delivery truck would appear problematic. The truck operator would need to ensure that the shear box was submerged under the concrete mix, rather than located upside down within the drum above the concrete mix, so that the sensors could operate properly. The truck operator might be required to shut down the engine to ensure that its vibration would not interfere with the operation of the vibration sensors on the shear box. Moreover, during transit, the shear box would likely be crushed by the tremendous weight of the aggregate, leading to repair problems. Having a shear box protruding within the mixing drum could also interfere with operation of the mixing paddles on the concrete mix.
Thus, an objective of the present invention is to monitor and to control the rheology of the concrete mix during transit from the ready-mix plant (or dispatch center) to the site of placement (or pouring), using the truck's mixing drum as a rheometer and monitoring equipment that is presently available in the concrete industry, without having to mount U-boxes or other pneumatic or vibratory devices within the drum.
Another objective of the present invention is to minimize the number of rheology factors that require assessment and to avoid having to analyze certain factors, such as vibration decay or vibratability, altogether.
A further objective of the present invention is to provide a method for controlling the rheology of highly flowable concretes, such as Self Consolidating Concrete (“SCC”). SCC is concrete that is able to flow and to consolidate under its own mass without vibration. SCC is highly filled, with typically about 70% aggregate by volume, as well as highly fluid. Due to this high degree of fluidity (characterized as “slump flow”), the horizontal flow (spread) rather than the vertical drop of the concrete placed in a slump cone is measured (ASTM C 1611-05). SCC typically exhibits 18-32 inch slump flow when measured by this slump cone method.
The prior art for estimating slump in a concrete mixer does not provide for estimation or monitoring of slump flow. Moreover, the use of the afore-mentioned slump cone method for measuring slump flow is not believed by the present inventors to provide an accurate means for assessing SCC or other highly fluid concrete mixes.
Hence, a novel method for monitoring and control of rheology of concrete mixes in a concrete delivery mixing truck is needed.